Syntheses and characterization of two oxoborates, (Pb3O)2(BO3)2MO4 (M=Cr, Mo)

Two oxoborates, (Pb3O)2(BO3)2MO4 (M=Cr, Mo), have been prepared by solid-state reactions below 700 deg. C. Single-crystal XRD analyses showed that the Cr compound crystallizes in the orthorhombic group Pnma with a=6.4160(13) A, b=11.635(2) A, c=18.164(4) A, Z=4 and the Mo analog in the group Cmcm with a=18.446(4) A, b=6.3557(13) A, c=11.657(2) A, Z=4. Both compounds are characterized by one-dimensional ∞1[Pb3O]4+ chains formed by corner-sharing OPb4 tetrahedra. BO3 and CrO4 (MoO4) groups are located around the chains to hold them together via Pb-O bonds. The IR spectra further confirmed the presence of BO3 groups in both structures and UV-vis diffuse reflectance spectra showed band gaps of about 1.8 and 2.9 eV for the Cr and Mo compounds, respectively. Band structure calculations indicated that (Pb3O)2(BO3)2MoO4 is a direct semiconductor with the calculated energy gap of about 2.4 eV. - Graphical abstract: (Pb3O)2(BO3)2MO4 (M=Cr, Mo) are characterized by 1D ∞1[Pb3O]4+ chains formed by corner-sharing OPb4 tetrahedra. BO3 and CrO4 (MoO4) groups are located around the chains to hold them together via Pb-O bonds.